Survey of mid-sized Venusian volcanoes using stereo-derived topography

Abstract

We have examined the morphology and morphometry of 21 mid-sized Venusian volcanoes (flow apron diameter 50–300 km) in the area covered by Magellan stereo imaging, excluding the already well-studied steep-sided domes. Volcanoes in this size range have received little previous attention because they are not adequately sampled by Magellan altimetry, but we have been able to discern their basic shapes with stereo-derived topography from multiple-look radar imagery. We grouped the volcanoes into five general categories based on morphology and slope: stellate fracture centers, shield-like, fissure-derived, concave-sided, and volcano/corona hybrids. Statistically, these constructs have slopes significantly lower than those of terrestrial and Martian volcanoes of similar diameter, as has been noted for large Venusian volcanoes, though some of the steepest constructs in this work have slopes similar to the shallowest terrestrial and Martian volcanoes; portions of the flanks of Ne Ngam Mons and an edifice that formed in the caldera of Toci Tholus exceed 9° and 20°, respectively. These 21 volcanoes formed in regions of middling crustal thicknesses and generally formed away from tessera terrains; they are over neither major mantle downwellings nor upwellings (with the possible exception of fissure-derived constructs). Similar constructs can be found at significantly larger diameters spanning over an order of magnitude in diameter, unlike other features such as steep-sided domes. The presence of these constructs over small mantle temperature anomalies and the existence of counterparts for each class at larger diameters suggest that a mantle heat source must be available to drive volcanism, which would explain why these volcanoes do not occur over areas of inferred downwelling. The volcanoes are not in areas of thickened crust, perhaps because it hinders magma ascent. The strength of the upwelling seems to have a significant effect on the size of the feature, with only large volcanoes over the highest negative mantle density anomalies. The varying morphologies reflect the local and regional stress regime and other crustal and lithospheric properties. We identified a correlation between mantle dynamics and base elevation: volcanoes in regions of inferred mantle upwelling tend to have higher base elevations than those in regions of inferred mantle downwelling, suggesting that modern base elevations are partly controlled by mantle dynamics. These mid-sized volcanoes are generally located high in the stratigraphic column, indicating that the majority of volcano forming events occurred late in the active portion of Venus' geologic history which is in agreement with previous works. Two volcanic constructs, a hybrid and a concave-sided volcano, appear to predate all but the earliest stratigraphic markers, which implies that, as a family, these volcanoes are not limited to some particular period in the observable surface history. We identified multiple volcano/corona hybrids, features with morphologies characteristic of both coronae and volcanoes. Often, these have topography incongruous from that inferred from radar images alone; for example, a topographic depression on the flank of a volcano with purely radial lava flows. This incongruous topography indicates significant changes in volcanic processes that may be useful in unraveling the time dependent evolution of the Venusian surface. The existence of these hybrids with flow diameters less than 100 km and their inferred deformation suggest that the elastic lithosphere must have been near negligible thickness during or following their formation. These hybrids form near regions of extension, which supports the hypothesis that a relatively thin elastic lithosphere is necessary to form them.